The unique combination of properties found in cellulose acetate, namely toughness, high impact strength, low flammability and ease of fabrication, have established its use in such areas as the coating, photographic and molding industries. Open or connected cell, foamed cellulose acetate structures are required in filters, e.g. cigarette filters, air filters for gasoline combustion engines, or in sponges or in any other article requiring an intercommunicating pore structure.
Polyolefin open cell structures having a peripheral skin are known, such structures being disclosed in U.S. Pat. Nos. 3,939,849 and 4,054,550. The polyolefin open cell structure is further characterized as a reticular structure wherein individual cells are bridged by fiber-like elements. Polyolefin foams when used in cigarette filter applications function by means of physical filtering alone; that is to say, mechanically trapping smoke-entrained matter. A major reason for the universal acceptance of cellulose acetate cigarette filters is the consumer acceptability of the taste imparted to the tobacco smoke stream as well as the ability to chemically sequester constituents from smoke vapor as well as mechanically trapping smoke-entrained matter, e.g. tarry substances.
Commercial cellulose acetate cigarette filters are high density filters (from about 0.14 to 0.15 grams per cubic centimeter) prepared from cellulose acetate tow. The manufacture of filter rods from tow is an expensive process involving manufacture of tow, opening of the tow bundle, precision topical application of plasticizer, forming the opened tow bundle into a rod, wrapping and gluing the rod and cutting the rod into suitable lengths.
U.S. Pat. No. 3,574,803 discloses a process for producing cellulose acetate structures. This process comprises mixing ripened cellulose acetate dope with a suitable diluent to its incipient precipitation point, dissolving an inert gas under pressure in the diluted cellulose acetate dope, precipitating the cellulose acetate from the gas-containing dope solution, and reducing the pressure on the mixture thereby allowing the gas to expand and escape and the cellulose acetate to foam. The foamed solid structure may then be washed and dried by conventional means.
U.S. Pat. No. 4,180,536 discloses that a low density open cell foamed cellulose acetate cigarette filter may be obtained by means of a process comprising melt extruding, through an extruder equipped with an eliptical or circular die, into a temperature conditioned atmosphere, a mixture of cellulose acetate, plasticizer, blowing agent and nucleating agent. The presence of a plasticizer is found to lower the glass transition temperature of cellulose acetate and make melt extrusion of a foamed cellulose acetate possible without undesirable thermal degradation of the polymer.